diff --git a/src/too/database.py b/src/too/database.py
index 1479f1d..d2d46be 100644
--- a/src/too/database.py
+++ b/src/too/database.py
@@ -135,6 +135,19 @@ def validate_too_targets(targets: polars.DataFrame):
             "At least one magnitude value is required."
         )
 
+    # Check that if one of the Sloan magnitudes is set, all are set.
+    sloan_mags = targets.select(polars.col("^[ugriz]_mag$"))
+    # Rows where at least one value is not null.
+    sloan_mags_not_null = sloan_mags.filter(
+        polars.fold(False, lambda a, b: a | b.is_not_null(), polars.all())
+    )
+    # Rows where at least one value is null.
+    sloan_mags_null = sloan_mags_not_null.filter(
+        polars.fold(False, lambda a, b: a | b.is_null(), polars.all())
+    )
+    if sloan_mags_null.height > 0:
+        raise ValidationError("Found rows with incomplete Sloan magnitudes.")
+
     if set(targets["fiber_type"].unique()) != set(["APOGEE", "BOSS"]):
         raise ValidationError(
             "Invalid fiber_type values. Valid values are 'APOGEE' and 'BOSS'."
diff --git a/src/too/mock.py b/src/too/mock.py
index 0026762..738ff11 100644
--- a/src/too/mock.py
+++ b/src/too/mock.py
@@ -217,7 +217,6 @@ def create_mock_too_catalogue(
         int(n_unknown * fraction_unknown_sdss),
     )
     sdss_unknown_targets = sdss_unknown_targets.with_columns(
-        polars.col(r"^[ugiz]_mag$").replace({}, default=None),
         catalogid=None,
         sdss_id=None,
     )
@@ -248,7 +247,11 @@ def create_mock_too_catalogue(
             "ra": numpy.random.uniform(0, 360, n_random),
             "dec": numpy.random.uniform(-90, 90, n_random),
             "gaia_g_mag": polars.Series(random_gaia, dtype=polars.Float32),
+            "u_mag": polars.Series(random_sdss, dtype=polars.Float32),
+            "g_mag": polars.Series(random_sdss, dtype=polars.Float32),
             "r_mag": polars.Series(random_sdss, dtype=polars.Float32),
+            "i_mag": polars.Series(random_sdss, dtype=polars.Float32),
+            "z_mag": polars.Series(random_sdss, dtype=polars.Float32),
         },
     )
 
diff --git a/tests/test_database.py b/tests/test_database.py
index 7f6dd2b..1ab4e47 100644
--- a/tests/test_database.py
+++ b/tests/test_database.py
@@ -105,6 +105,7 @@ def test_validate_too_target_passes(too_mock: polars.DataFrame):
         ("active", "Null 'active' column values found"),
         ("mag_columns", "ToOs found with missing magnitudes"),
         ("fiber_type", "Invalid fiber_type values."),
+        ("sloan_mags", "Found rows with incomplete Sloan magnitudes."),
     ],
 )
 def test_validate_too_target_fails(
@@ -133,6 +134,10 @@ def test_validate_too_target_fails(
         )
     elif test_mode == "fiber_type":
         too_mock_test[0, "fiber_type"] = "INVALID"
+    elif test_mode == "sloan_mags":
+        too_mock_test = too_mock_test.with_columns(
+            u_mag=polars.lit(None, dtype=polars.Float32)
+        )
 
     with pytest.raises(ValidationError, match=error_message):
         validate_too_targets(too_mock_test)